In a cellular radio system mobile stations connect to a base station in order to establish a connection with the cellular radio system. As a part of the procedure to establish the connection the mobile station goes from an idle mode to an active mode.
This procedure, usually termed call is setup, includes the step of identifying the base station currently best suited to handle the connection. The base station best suited is normally the base station currently providing the strongest signal to the mobile station. Hence during a typical call setup, the mobile station retrieves information about the base station that provides the strongest signal.
The 3GPP standard for GSM requires that the mobile station when in idle mode decodes the Base Station Identity Code BSIC information which is broadcast on the SCH channel. When the mobile station is in idle mode it should decode the BSIC every 30 seconds in order to i.a. keep track of the base station providing the strongest signal, see also 3GPP spec. 45 008.
Moreover, the mobile in idle mode shall continue to monitor all BCCH carriers as indicated by the BCCH allocation (BA-list). This is done by measuring and running average of received signal level (RLA_C) which is updated once every Max {5 s, five consecutive paging blocks of that MS}. At least 5 received signal level measurement samples are required per RLA_C value.
This means that in a worst case only 5 measurements per frequency is performed during the RLA_C period which can be up to typically 10 seconds. This time period is defined in 45.008 accordingly:
Consequently it is a likely scenario that the signal strength measurements will only be updated 5 times every 10th second per frequency.
One reason for not performing more frequent BSIC decoding and signal strength measurements is that this would increase the power consumption and hence reduce the battery life time of the mobile station.
Due to the relatively relaxed requirement on the BSIC decoding frequency, i.e. once every 30 seconds and the relaxed requirement on the signal strength measurements, a mobile station may not set up the call during call set-up using the base station providing the strongest signal. The problem becomes even worse if the mobile station is traveling at a high speed, such as when a person is riding on a train or in a bus, because then the best base station may be shifting at a high rate indeed.
If a mobile station tries to set up a call to base station not providing the strongest signal, then there is a risk that the call could be dropped. This is particularly the case when the mobile station is traveling away from a base station that the mobile station tries to set up the connection to.
Hence, there exist a need for a mobile station and a method that minimizes the risk for dropping a call during a call set-up, and which mobile station and method does not increase the power consumption in the mobile stations.